Here's a stab at it.

==========Script follows ===========
#!/bin/ksh
filename=$1
tline=$(wc -l $filename | awk '{print $1}')
let hline=$tline/2
split -l $hline $filename
=========== End of script ==========

This will output 2 files, call xaa and xab by default, check the man page for split to see how to make it what you want.

Syntax to execute it is scriptname [file_to_be_split]

Variable definitions
filename - file to be split
tline - number of lines in the file
hline - 1/2 of the number of lines in the file

Hope this helps, sure there are other ways.